The molecular basis of the antidepressant action of the magic mushroom extract, psilocin

Magic mushrooms, and their extract psilocybin, are well-known for their psychedelic properties and recreational use. Psilocin, the bio-active form of psilocybin, can potentially treat various psychiatric diseases. Psilocin putatively exerts its psychedelic effect as an agonist to the serotonin 2A receptor (5-HT2AR), which is also the receptor for the neurological hormone serotonin. The two key chemical differences between the two molecules are first, that the primary amine in serotonin is replaced with a tertiary amine in psilocin, and second, the hydroxyl group is substituted differently on the aromatic ring. Here, we find that psilocin can bind to 5-HT2AR with an affinity higher than serotonin, and provide the molecular logic behind the higher binding affinity of psilocin using extensive molecular dynamics simulations and free energy calculations. The binding free energy of psilocin is dependent upon the protonation states of the ligands, as well as that of the key residue in the binding site: Aspartate 155. We find that the tertiary amine of psilocin, and not the altered substitution of the hydroxyl group in the ring is responsible for the increased affinity of psilocin. We propose design rules for effective antidepressants based on molecular insights from our simulations.

Targeting 5-HT2A receptors and Kv7 channels in PFC to attenuate chronic neuropathic pain in rats using a spared nerve injury model

Chronic pain remains a disabling disease with limited therapeutic options. Pyramidal neurons in the prefrontal cortex (PFC) express excitatory G_q-coupled 5-HT_2A receptors (5-HT_2AR) and their effector system, the inhibitory Kv7 ion channel. While recent publications show these cells innervate brainstem regions important for regulating pain, the cellular mechanisms underlying the transition to chronic pain are not well understood. The present study examined whether local blockade of 5-HT_2AR or enhanced Kv7 ion channel activity in the PFC would attenuate mechanical allodynia associated with spared nerve injury (SNI) in rats. Following SNI, we show that inhibition of PFC 5-HT_2ARs with M100907 or opening of PFC Kv7 channels with retigabine reduced mechanical allodynia. Parallel proteomic and RNAScope experiments evaluated 5-HT_2AR/Kv7 channel protein and mRNA. Our results support the role of 5-HT_2ARs and Kv7 channels in the PFC in the maintenance of chronic pain.

Gs signaling pathway distinguishes hallucinogenic and nonhallucinogenic 5-HT2AR agonists induced head twitch response in mice

5- HT2A receptor is a member of the family A G-protein-coupled receptor. It is involved in many psychiatric disorders, such as depression, addiction and Parkinson's disease. 5-HT2AR targeted drugs play an important role in regulating cognition, memory, emotion and other physiological function by coupling G proteins, and their most notable function is stimulating the serotonergic hallucination. However, not all 5-HT2AR agonists exhibit hallucinogenic activity, such as lisuride. Molecular mechanisms of these different effects are not well illustrated. This study suggested that 5-HT2AR coupled both Gs and Gq protein under hallucinogenic agonists DOM and 25CN-NBOH stimulation, but nonhallucinogenic agonist lisuride and TBG only activates Gq signaling. Moreover, in head twitch response (HTR) model, we found that cAMP analogs 8-Bromo-cAMP and PDE4 inhibitor Rolipram could increase HTR, while Gs protein inhibitor Melittin could reduce HTR. Collectively, these results revealed that Gs signaling is a key signaling pathway that may distinguish hallucinogenic agonists and nonhallucinogenic agonists.

Involvement of 5-HT2 serotonin receptors in cognitive defects induced by aristolochic acid I in mice

Aristolochic acids (AAs) are a natural bioactive substance found in Chinese herbs, which are widely used for treating diseases. Many studies have demonstrated that AAs have various pharmacological function, while increasing reports indicated its toxicity. However, the role AAs in cognition remains poorly understood. This study explored the neurotoxic effect of aristolochic acid I (AAI), the most toxic component of the AAs family, on hippocampal synaptic plasticity and spatial cognition in mice. C57BL/6 mice were exposed to 5 mg/kg AAI for 4 weeks. After chronic treatment, AAI considerably increased the level of anxiety and the degree of behavioral despair in mice. Working and reference error rates were higher in the AAI exposed mice than in the control. This was further validated by the molecular docking studies, which AAI might interact with 5-HT₂ serotonin receptor (5-HT_2AR). Mechanism investigation indicated that AAI triggered inflammation in the hippocampus of mice through increasing the activity of Tnf-α-NF-κB-IL-6 signaling pathway. Conclusively, chronic AAI administration causes inflammation, and it possibly also serves as a potential antagonist of 5-HT_2AR to influence the cognition function in C57BL/6 mice.

[5-HT(2A) receptor/protein kinase C pathway in mediating long-term facilitation of carotid sinus nerve discharge in chronic intermittent hypoxia rats]

Objective: To explore the role of 5-HT(2A)R/PKC pathway in mediating long-term facilitation (LTF) of carotid sinus nerve (CSN) discharge in chronic intermittent hypoxia (CIH) rats. Methods: With number table, 24 adult SD rats were randomly divided into saline control group (group A, n=6), 5-HT(2A)R antagonist (ketanserin) group (group B, n=6), PKC inhibitor (PKC θ-pseudosubstrate) group (group C, n=6) and combined ketanserin with PKC θ-pseudosubstrate group (group D, n=6). All rats were placed into the animal chambers for CIH treatment, 8 h per day (from 9: 00 to 17: 00) for 4 consecutive weeks. 28 days later, 5 min × 3 times of stimulation with acute intermittent hypoxia (AIH) were given, after that, stable CSN discharge activities were recorded and compared before and after intravenous injection of saline (group A), ketanserin (group B), PKC θ-pseudosubstrate (group C) or ketanserin + PKC θ-pseudosubstrate (group D), respectively. Results: There were no significant difference in the baseline (before AIH stimulation) average peak amplitude of CSN discharge among the four groups (P>0.05). In group A, the amplitude of CSN discharge at 30 min and 60 min after AIH were (5.01 ± 0.53) μV and (4.95 ± 0.34) μV respectively, which were significantly higher than that before AIH (P<0.01). The results implied that the CSN LTF could be induced by AIH in CIH pre-treatment rats. In group B, the amplitude of CSN discharge at 30 min and 60 min after AIH were (3.79 ± 0.42) μV and (3.73 ± 0.46) μV, respectively, which were still significantly higher than that before AIH (P<0.01), showing that carotid sinus nerve LTF couldn't be completely blocked by 5-HT(2A)R antagonist in rats. After injection of PKC θ-pseudosubstrate or ketanserin + PKC θ-pseudosubstrate in group C or D, there were no significant differences in CSN discharge amplitude before and after AIH (P>0.01), suggesting that inhibition of PKC alone or 5-HT(2A)R/PKC pathway could completely block the LTF of CSN. Conclusion: 5-HT(2A)R/PKC pathway was involved in mediating long-term facilitation of carotid sinus nerve discharge in CIH rats.

Effects of olanzapine and betahistine co-treatment on serotonin transporter, 5-HT2A and dopamine D2 receptor binding density

Olanzapine is widely used in treating multiple domains of schizophrenia symptoms but induces serious metabolic side-effects. Recent evidence has showed that co-treatment of betahistine (a histaminergic H1 receptor agonist and H3 receptor antagonist) is effective for preventing olanzapine-induced weight gain/obesity, however it is not clear whether this co-treatment affects on the primary therapeutic receptor binding sites of olanzapine such as serotonergic 5-HT2A receptors (5-HT2AR) and dopaminergic D2 receptors (D2R). Therefore, this study investigated the effects of this co-treatment on 5-HT2AR, 5-HT transporter (5-HTT) and D2R bindings in various brain regions involved in antipsychotic efficacy. Female Sprague Dawley rats were administered orally (t.i.d.) with either olanzapine (1mg/kg), betahistine (2.7 mg/kg), olanzapine plus betahistine (O+B), or vehicle (control) for 2 weeks. Quantitative autoradiography was used to detect the density of [(3)H]ketanserin, [(3)H]paroxetine and [(3)H]raclopride binding site to 5-HT2AR, 5-HTT and D2R. Compared to the controls, olanzapine significantly decreased [(3)H]ketanserin bindings to 5-HT2AR in the prefrontal cortex, cingulate cortex, and nucleus accumbens. Similar changes in 5-HT2AR bindings in these nuclei were also observed in the O+B co-treatment group. Olanzapine also significantly decreased [(3)H]paroxetine binding to 5-HTT in the ventral tegmental area and substantia nigra, however, both olanzapine only and O+B co-treatment did not affect [(3)H]raclopride binding to D2R. The results confirmed the important role of 5-HT2AR in the efficacy of olanzapine, which is not influenced by the O+B co-treatment. Therefore, betahistine co-treatment would be an effective combination therapy to reduce olanzapine-induced weight gain side-effects without affecting olanzapine's actions on 5-HT2AR transmissions.

[¹⁸F]Altanserin and small animal PET: impact of multidrug efflux transporters on ligand brain uptake and subsequent quantification of 5-HT₂A receptor densities in the rat brain

INTRODUCTION

The selective 5-hydroxytryptamine type 2a receptor (5-HT(2A)R) radiotracer [(18)F]altanserin is a promising ligand for in vivo brain imaging in rodents. However, [(18)F]altanserin is a substrate of P-glycoprotein (P-gp) in rats. Its applicability might therefore be constrained by both a differential expression of P-gp under pathological conditions, e.g. epilepsy, and its relatively low cerebral uptake. The aim of the present study was therefore twofold: (i) to investigate whether inhibition of multidrug transporters (MDT) is suitable to enhance the cerebral uptake of [(18)F]altanserin in vivo and (ii) to test different pharmacokinetic, particularly reference tissue-based models for exact quantification of 5-HT(2A)R densities in the rat brain.

METHODS

Eighteen Sprague-Dawley rats, either treated with the MDT inhibitor cyclosporine A (CsA, 50 mg/kg, n=8) or vehicle (n=10) underwent 180-min PET scans with arterial blood sampling. Kinetic analyses of tissue time-activity curves (TACs) were performed to validate invasive and non-invasive pharmacokinetic models.

RESULTS

CsA application lead to a two- to threefold increase of [(18)F]altanserin uptake in different brain regions and showed a trend toward higher binding potentials (BP(ND)) of the radioligand.

CONCLUSIONS

MDT inhibition led to an increased cerebral uptake of [(18)F]altanserin but did not improve the reliability of BP(ND) as a non-invasive estimate of 5-HT(2A)R. This finding is most probable caused by the heterogeneous distribution of P-gp in the rat brain and its incomplete blockade in the reference region (cerebellum). Differential MDT expressions in experimental animal models or pathological conditions are therefore likely to influence the applicability of imaging protocols and have to be carefully evaluated.